Notice 2 things:
- “Smart indent lists: automatically set indentation and place list items correctly” is turned on.
- When the cursor is in the middle (in contrast to the beginning) of a line of a list item, pressing tab adds indentation.
Given these two observations, I was very surprised that pressing tab indents by 2 spaces rather than by whatever amount is needed to reach the next indentation level. Moreover, I want it to indent by whatever amount is needed to reach the next indentation level.